1. Field of the Invention
This invention relates to a radiation image recording apparatus for recording a radiation image of an object. This invention particularly relates to a radiation image recording apparatus for recording a radiation image of an object on a stimulable phosphor sheet for storing radiation energy so that an arbitrary tomographic image of the object may later be reconstructed.
2. Description of the Prior Art
As a method of obtaining a tomographic image of a desired tomographic layer of an object such as the human body, tomography has heretofore been known. In tomography, a radiation source such as an X-ray tube and a radiographic film are positioned with the object intervening therebetween and moved relative to each other at the time of exposure to a radiation around an arbitrary tomographic layer of the object so that the linear rule (specifying that the focal point of the radiation source, a point on the tomographic layer and a point on the radiographic film are positioned in a straight line) and the geometric rule (specifying that the ratio of the distance between the focal point of the radiation source and the tomographic layer to the distance between the tomographic layer and the radiographic film is maintained constant) are satisfied. In this manner, only an image of a desired tomographic layer is formed on the radiographic film, and images of the other tomographic layers of the object are blurred. As a result, a radiation image of only the desired tomographic layer of the object is obtained. In the tomography, it is only necessary that the linear rule and the geometric rule are satisfied, and movements of the radiation source and the radiographic film may be conducted along any path, for example, a linear path, a circular path, an elliptic path or a spiral path.
However, in the aforesaid tomography, only the tomographic image of a single tomographic layer of the object may be recorded by a single radiation image recording step. Accordingly, an attempt has been made to record radiation images of an object respectively on a plurality of radiographic films by emitting a radiation to the object in different directions, and integrating image signals detected from the radiographic films, thereby reconstructing a tomographic image of an arbitrary tomographic layer of the object. This method is called tomosynthetic reconstruction. The basic principle of this method will hereinbelow be described with reference to FIGS. 4A and 4B. As shown in FIG. 4A, a radiation source 10 constituted by an X-ray tube or the like is sequentially moved to positions R1, R2 and R3 while facing an object 11. A radiation 12 is emitted by the radiation source 10 to the object 11 at the respective positions R1, R2 and R3, and radiation images of the object 11 are respectively recorded on radiographic films F1, F2 and F3. During this operation, the radiation source 10 and each of the radiographic films F1, F2 and F3 are positioned to satisfy the linear rule and the geometric rule with respect to a point O within the object 11 or in the vicinity thereof. Therefore, images of the point O and an arbitrary point P within the object 11 are recorded on the radiographic films F1, F2 and F3 at positions as shown in FIG. 4B. Namely, the image of the point O is recorded at the same positions on the films F1, F2 and F3. On the other hand, the image of the point P spaced from the point O is recorded at different positions on the films F1, F2 and F3. Specifically, the image of the point P is shifted by .DELTA.s from the image of the point O on the film F1, coincides therewith on the film F2, and is shifted by -.DELTA.s therefrom on the film F3.
Accordingly, by shifting the image signals detected from the films F1 and F3 respectively by .DELTA.s and -.DELTA.s and integrating the shifted image signals with the image signal detected from the film F2, it is possible to obtain an image signal for reconstructing the image of only the tomographic layer T on which the point P lies. The shift value .DELTA.s is given as a function of an irradiation angle .theta. of the radiation 12 with respect to the object 11. Though on the drawing sheet in FIG. 4A the radiation source 10 is shown as moved along a circular arc path, it may also be moved along a circular path normal to the drawing sheet in FIG. 4A while at the same time the radiographic films F1, F2 and F3 are moved along a circular path normal to the drawing sheet. In this case, the image of the point P is shifted two-dimensionally with respect to the image of the point O on the films F1, F2 and F3. Therefore, the image signals detected from the films F1 and F3 should be two-dimensionally shifted in the aforesaid integration.
However, in the aforesaid conventional method of reconstructing a tomographic image, radiation images are recorded on radiographic films. Therefore, it is necessary to detect the images on the radiographic films by use of a photometer or the like and to obtain the image signal for reconstruction of a tomographic image. This operation is very troublesome. In order to eliminate the troublesome operation, an attempt has been made to use an image intensifier (I.I.) as the radiation detector instead of the radiographic film, to record the radiation image formed on the I.I. by use of a television camera, and to obtain an image signal. However, with this method, resolution of the reconstructed tomographic image is low, and distortion becomes large.